1. Birkeland Centre for Space Science, Department of Physics and Technology; University of Bergen; Bergen Norway
2. University Centre in Svalbard; Longyearbyen Norway
3. Birkeland Centre for Space Science; University Centre in Svalbard; Longyearbyen Norway
4. Now at British Antarctic Survey; Cambridge UK
5. The Johns Hopkins University Applied Physics Laboratory; Laurel Maryland USA
6. Department of Physics; University of Oslo; Oslo Norway